Electrical conductors used for transmitting signals that may be susceptible to external electromagnetic noise often employ a center conductor surrounded by a conductive shield. The shield is grounded to prevent external electric fields from influencing the signal on the central conductor. A common xe2x80x9ccoaxial cablexe2x80x9d shielded conductor, used for radio-frequency signals, employs a braided shield surrounding a central multi-strand conductor separated from the braided shield by an insulator of predetermined diameter and dielectric properties. The braded shield is surrounded in turn by a second insulator that protects the shield from damage or electrical contact with other conductors.
In applications where there are intense external electrical/magnetic fields, for example, in magnetic resonance imaging (MRI), significant current may be induced in the shield causing failure of the shielding effect and possibly damage to the shield and its adjacent insulation from heating. One method of reducing shield current is with ferrite xe2x80x9cbeadsxe2x80x9d which fit over the shield to resistively damp eddy currents induced by the shield currents. It is also known to reduce such shield currents by creating an S-trap in which the coaxial cable is wound in a first direction and then optionally a second direction about a cylindrical form to produce a self-inductance among the coils of each winding set. A capacitance is connected in parallel with the inductance (by attaching leads of a capacitor to the shield at separated points in each winding) providing parallel resonant circuits tuned to the particular frequency of the offending external radio frequency field. The tuning provides the shield with a high impedance at the frequency of the interference, resisting current flow at this frequency, while the counter-winding reduces inductive coupling of the trap to the noise.
While the S-trap may successfully reduce current flow in the shield, it requires additional cable length for the windings and thus may contribute to a loss of signal strength and may introduce an undesirable phase change in the signal. Further, manufacture of the S-trap is cumbersome, requiring modification of the coaxial cable including a removal of portions of its external insulation for attachment of a capacitor. The fixed position of the S-trap makes it difficult to adjust the S-trap to a location on the shield having maximum current, as is desirable. Ferrite beads are unsuitable in areas of intense magnetic fields, such as are found in magnetic resonance imaging machines.
The present invention reduces currents in the shield of a cable (for example, a coaxial cable) without electrical attachment to the cable. The invention therefore may be easily added (or removed) from the shielded cable and adjusted in position along the length of the cable. The invention does not increase the length of the cable and therefore does not increase cable losses and undesired phase changes caused by increased cable length.
Specifically, the invention provides a shield current trap having a first and second concentric tubular conductor electrically connected to provide a resonance-induced high impedance to current flow in a path consisting of the inner and outer conductors and their junctions.
The summary in this application is not intended to define the scope of the invention for which purpose claims are be provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment also does not define the scope of the invention and reference must be made therefore to the claims for this purpose.